1. Field of the Invention
The present invention relates to a pin-type photovoltaic device and a photoelectric transducer using non-single crystal silicon such as amorphous silicon, microcrystalline silicon or polycrystalline silicon. It also relates to a method of manufacturing such a photovoltaic device and such a photoelectric transducer.
2. Related Background Art
The plasma CVD method typically using radio-frequency (RF) electric waves of 13.56 MHz has been popularly known in the manufacture of pin-type photovoltaic devices, such as solar cells, and photoelectric transducers, such as photosensors, using amorphous silicon film. However, with the plasma CVD method using 13.56 MHz, it has been realized that the quality of the produced film is disproportionally degraded when the speed of producing the thin film is raised. This means that it is difficult to increase the throughput in order to make the method adapted to mass production.
The plasma CVD method typically using microwaves (MW) of 2.45 GHz has been utilized as an alternative that can produce relatively high quality thin film if the thin film producing speed is raised. For example, an i-type semiconductor layer prepared by a microwave plasma CVD method is reported in Kazufumi Azuma, Takeshi Watanabe and Juichi Shimada, "a-Si Solar Cells Prepared by Using a Microwave Plasma CVD Method";
Preliminary Papers for the 50th Applied Physics Society Lecture Meeting, pp. 566.
A thin film photovoltaic device using amorphous silicon thin film typically has a pin structure and its i-type semiconductor layer is principally responsible for photoelectric conversion. FIG. 1 of the accompanying drawings schematically shows a known typical pin-type photovoltaic device comprising a substrate 101, an n-type (or p-type) semiconductor layer 102, an i-type semiconductor layer 103, a p-type (or n-type) semiconductor layer 104, a transparent electrode 105 and a collector electrode 106.
Numerous efforts have been made to improve the junction characteristics of thin film photovoltaic devices by using microcrystals for the p-type and n-type semiconductor layers. For example, Japanese Patent Application Laid-Open No. 57-187971 discloses a method in which an i-type semiconductor layer comprises amorphous silicon, wherein the improvement in the output current and the output voltage of the device lies in using microcrystalline silicon with an average grain size of less than 100 angstroms at least for the p-type semiconductor layer or the n-type semiconductor layer located on the light receiving side of the device.
However, pin-type solar cells using amorphous silicon for the i-type semiconductor layer can be accompanied by the undesired phenomenon (referred to as Staebler-Wronski effect) of an increased flaw density in the i-type semiconductor layer that gives rise to a reduced photoelectric conversion efficiency when irradiated with light. Such a harmful phenomenon is vital in practical applications.
In recent years, efforts have been paid to use i-type microcrystalline silicon for the photoelectric conversion layer of amorphous silicon type thin film photovoltaic devices. Pin-type solar cells comprising an i-type semiconductor layer of microcrystalline silicon are advantageous in that they are not degraded by light. For example, Shah et al. of Neufchatel University reported at the 25th IEEE PV Specialists Conference, Washington, May 13-17, 1996 a pin-type solar cell comprising a p-type semiconductor layer, an i-type semiconductor layer and an n-type semiconductor layer, all of which are made of microcrystalline silicon, that shows a photoelectric conversion efficiency of 7.7% and is not degraded by light. While Shah et al. use a high frequency wave plasma CVD method that is essentially same as any known methods for preparing a microcrystalline silicon i-type semiconductor layer, they also use a VHF band frequency of 110 MHz to generate plasma.
On the other hand, according to the report by Shah et al. of Neufchatel University, they realized a deposition rate of 1.2 angstroms per second for a microcrystalline silicon i-type semiconductor layer having a thickness of 3.6 .mu.m. By a rule of thumb, it is clear that it takes more than 8 hours to form a microcrystalline silicon i-type semiconductor layer with such a method. While a solar cell having such an i-type semiconductor layer shows a high photoelectric conversion efficiency and is free from degradation by light, the throughput will be extremely poor in manufacturing and hence it will be very difficult to manufacture such solar cells feasibly at low cost.
In order to manufacture pin-type solar cells comprising a microcrystalline silicon i-type semiconductor layer on a realistic mass production basis, it is absolutely necessary to dramatically raise the rate of forming a microcrystalline silicon i-type semiconductor layer from the currently available level. However, it has been evidenced by a number of studies that, when the rate of forming amorphous silicon or microcrystalline silicon is simply raised in a conventional film-forming process, the produced film shows a poor quality due to inhibition of lattice relaxation at an outer most surface of the produced film.
It is true that lattice relaxation can be promoted by raising the temperature of the base member. Then, however, the dopant such as phosphor (or boron) in the n-type semiconductor layer (or the p-type semiconductor layer) prepared prior to the i-type semiconductor layer can be diffused into the i-type semiconductor layer to damage the quality of the layer and consequently the operating characteristics of the prepared solar cell. It is believed that i-type microcrystalline silicon produced by high frequency plasma CVD is inherently a weak n-type and hence any diffusion of n-type dopants such as phosphor into i-type microcrystalline silicon should be suppressed.
While microcrystalline silicon is promising if compared with amorphous silicon because the former shows a higher photostability than the latter, known microcrystalline silicon have disadvantages that have to be dissolved.